The transient spectrometer (see below) has been used to systematically examine the kinetics of photolysis-induced reactions of several heme-protein complexes. Complete data have been collected and analyzed for the carbon monoxide complexes of myoglobin and hemoglobin. In addition, preliminary data have been collected on several other systems. The results for hemoglobin reveal a sequence of ligand binding and spectral events which are tentatively interpreted as (1) rebinding of protein-bound ligand which has been dissociated from the heme (10 to the seven sec); (2) escape of ligand from the protein (10 to the seven sec); (3) tertiary structural relaxation (10 to the six sec); (4) quaternary structural relaxation (2x10 to the five sec); and (5) recombination of ligand from solution to the mixture of R (6 x 10 to the six M sec); and (5) recombination of ligand from solution to the mixture of R (6 x 10 to the sixM sec) and T quaternary states (20 to the five sec). These results provide an outline in which to examine the ligand binding reaction to heme proteins in terms of elementary steps. The present data characterize the path for ligand dissociation and provide overall rates for the rebinding processes.